Reference voltage generators are used in electronic systems such as voltage regulators that operate based on a constant reference voltage. Various attempts have been made to provide a stable voltage reference independent of fabrication process, supply voltage, and ambient temperature variations.
One known method for reference voltage generation uses field effect transistors (FETs) with heavily doped gate structures, where a reference voltage is obtained as a difference in threshold voltage between a pair of FET devices. FIG. 1 is a circuit diagram illustrating an example of such a FET-based reference voltage generator 100.
As shown in FIG. 1, the reference voltage generator 100 includes n-channel FETs Ma, Mb, and Mc connected in series between a supply voltage Vcc and a ground voltage GND. The transistors Mb and Mc are coupled at an output node Lxout.
In the reference voltage generator 100, the transistor Ma has a drain connected to Vcc, a gate and source connected together, and a substrate gate connected to GND. The transistor Mb has a drain connected to the source of the transistor Ma and a gate, source, and substrate gate connected to the output node Lxout. The transistor Mc has a gate and drain connected to the output node Lxout and a source and substrate gate connected to GND.
The n-FETs Ma, Mb, and Mc are each formed in a p-well of an n-type semiconductor substrate. The transistor Ma is a depletion mode transistor. The n-FETs Mb and Mc have heavily doped gate structures, n-type for the transistor Mb and p-type for the transistor Mc, with different threshold voltages but substantially uniform substrate impurity and channel doping concentrations.
When supplied with Vcc, the reference voltage generator 100 outputs a reference voltage Vref at the output node Lxout, where the depletion mode transistor Ma serves to stabilize the voltage supplied to the series transistors Mb and Mc, and the transistor Mb acts as a constant current source so that the series transistors Ma, Mb, and Mc conduct a same drain current.
In such a configuration, the reference voltage Vref is given by the following equation.Vref=VthMC−(KMb/KMc)1/2*VthMb  Equation (a)
where KMb represents device transconductance for Mb, KMc represents device transconductance for Mc, VthMb represents threshold voltage of Mb, and VthMc represents threshold voltage of Mc.
When the transistors Mb and Mc have equal transconductance, i.e., KMb=KMc, Equation (a) can be rewritten as follows:Vref=VthMc−VthMb  Equation (b)
In this case, the reference voltage Vref equals the difference between VthMb and VthMc, which is in turn equal to the difference in gate-to-source voltage Vgs between the series transistors Mb and Mc. By setting an identical transconductance parameter for the series transistors Mb and Mc, the reference voltage generator 100 generates a constant output which is less sensitive to fabrication process and supply voltage variations.
Additionally, the reference voltage generator 100 achieves good thermal stability in a configuration where the transistors Mb and Mc have aspect ratios (i.e., width-to-length ratios of the channel) adjusted with respect to each other. The reference voltage generator 100 with such a configuration can generate a reference voltage substantially independent of fabrication process, supply voltage, and ambient temperature variations.
Referring to FIG. 2, a plot of the reference voltage Vref versus the supply voltage Vcc in the reference voltage generator 100 is described.
As shown in FIG. 2, the reference voltage Vref becomes constant at approximately 1 volt when the supply voltage Vcc is sufficient. Naturally, this 1-volt output voltage requires a corresponding input voltage for proper functioning of the circuit.
FIG. 3 shows simulated plots of the drain current id versus the drain-to-source voltage Vd for the transistors Mb and Mc, respectively, where the intersection of the two I-V curves indicates an operating point of the reference voltage generator 100, which is approximately 1 volt.
As can be seen from FIGS. 2 and 3, a supply voltage of at least 1 volt is required to drive the reference voltage generator 100, which does not meet today's low voltage and low power consumption requirements. Hence, what is needed is a reference voltage generator that is operable at low voltages, and can generate a reference voltage substantially independent of fabrication process, supply voltage, and ambient temperature variations.